The proposed research will complete the first international longitudinal twin study of genetic and environmental influences on pre-reading and early reading-related skills from preschool through the end of kindergarten, 1st, and 2nd grades, when children "learn to read", and at the end of 4th grade, when children "read to learn." Preschool data from measures of attention, phonological awareness, rapid naming, verbal memory, vocabulary, grammar, morphology, and print knowledge are being used to form reliable composite scores and latent traits for concurrent and developmental behavior-genetic analyses. Over the next 5 years, we propose to complete follow-up assessments of reading and related skills for 568 twin pairs through the end of 2nd grade, and extend the longitudinal design to the end of 4th grade with increased emphasis on the twins'reading and listening comprehension, and on their print exposure. Environmental measures related to the twins'reading and language development are included to help identify specific factors that contribute to behavior-genetic estimates of influences from shared and non-shared environment, and to identify potentially important genotype-environment correlations in reading and language development. The twin data from the Colorado study will be compared and combined where appropriate with twin data from parallel studies in Australia and Scandinavia for more powerful analyses of individual differences and disabilities in reading and related skills. Previous twin research with older children and adults has shown strong genetic and weaker environmental influences on both individual differences and group deficits in reading and related skills, but the developmental pathways for these influences and their implications for intervention are not well understood. Completion of the proposed longitudinal twin study from preschool through the 4th grade will provide important new evidence regarding the genetic and environmental etiology of individual differences and disabilities across this critical period of early reading development.